The present invention provides a novel process for the preparation of isoolefin copolymers in the presence of zirconium halides and/or hafnium halides and organic acid halides, in particular for the preparation of higher isoprene-containing butyl rubbers, as well as isoolefin copolymers constructed of isobutene, isoprene and optionally further monomers.
The process currently used for the preparation of butyl rubber is known, for example, from Ullmann""s Encyclopedia of Industrial Chemistry, Vol. A 23, 1993, pp. 288 to 295. The cationic copolymerization of isobutene with isoprene in the slurry process with methyl chloride as the process solvent is carried out with aluminum trichloride as an initiator with the addition of small quantities of water or hydrogen chloride at xe2x88x9290xc2x0 C. The low polymerization temperatures are necessary in order to obtain molecular weights sufficiently high for rubber applications.
It is in principal possible to compensate for the molecular weight-lowering (=regulating) effect of the dienic comonomers by even lower reaction temperatures. However, in this case there is a more marked occurrence of the side reactions which lead to gel formation. Gel formation at reaction temperatures of around xe2x88x92120xc2x0 C. and possible ways of reducing it are described (q.v. W. A. Thaler, D. J. Buckley, Sr., Meeting of the Rubber Division, ACS, Cleveland, Ohio, May 6th to 9th, 1975, published in Rubber Chemistry and Technology 49, 960 to 966 (1976)). On the one hand, of the auxiliary agents which are necessary here, such as CS2, is difficult, and they must furthermore be utilized at relatively high concentrations.
The gel-free copolymerization of isobutene with various comonomers at temperatures of around xe2x88x9240xc2x0 C. with the use of preformed vanadium tetrachloride to obtain products having molecular weights sufficiently high for rubber applications is additionally known (EP-A1-0 818 476).
U.S. Pat. No. 2,682,531 describes zirconium tetrachloride-ether complexes and the use thereof as catalysts for the polymerization of, inter alia, isoolefins. It is emphasized in column 2, line 20 et seq. that the use of zirconium tetrachloride alone leads to unsatisfactory results. The ether which is preferably used is xcex2,xcex2xe2x80x2-dichloroethyl ether, a carcinogen. The diphenyl ether which is likewise listed as an example results in poorly soluble complexes which have sufficient activity only at very high dosing levels. Diethyl ether (named specifically in the patent as a possible ether) results in completely ineffective complexes.
The older application DE-A-100 42 118 describes a process for the preparation of isoolefin copolymers with the use of initiator systems prepared from zirconium halides or hafnium halides in the presence of organic nitro compounds. While these initiator systems permit the preparation of highly unsaturated butyl rubbers, for example, they have the disadvantage that it is very difficult in practice to use organic nitro compounds on a large industrial scale on account of the associated explosion hazard.
The object of the present invention was to provide a process for the preparation of high molecular weight low-gel isoolefin copolymers, in particular, for the preparation of butyl rubbers having more than 2% isoprene in the polymer without the use of nitro compounds.
The present invention provides a process for the preparation of high molecular weight isoolefin copolymers in the presence of zirconium halides and/or hafnium halides, wherein the polymerization takes place in the presence of organic acid halides.
The process is preferably utilized with isoolefins having 4 to 16 carbon atoms and dienes which are copolymerizable with the isoolefins, optionally in the presence of further monomers which are copolymerizable with the monomers. More preferably, isobutene and isoprene are utilized, optionally in the presence of further monomers which are copolymerizable with these.
The process is preferably carried out in a solvent which is suitable for cationic polymerization, such as halogenated and non-halogenated hydrocarbons or mixtures thereof, in particular chloroalkanes and chloroalkane/alkane mixtures, more preferably, methyl chloride and methylene chloride or mixtures thereof with alkanes.
The zirconium halide and/or hafnium halide is preferably mixed with the organic acid halide in the absence of the monomer.
The organic acid halides which are utilized are commonly known and are available generally. The acid halides preferably used according to the present invention are defined by the general formula (I)
R-COXxe2x80x83xe2x80x83(I), 
wherein R is selected from the group of C1-C18-alkyl, C3-C18-cycloalkyl and C6-C24-cycloaryl.
C1-C18-alkyl is understood to mean any of the linear or branched alkyl radicals having 1 to 18 C atoms, which are known to those skilled in the art, such as methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, n-pentyl, i-pentyl, neopentyl, hexyl and the further homologues, which may for their part be in turn substituted. Here, alkyl, as well as cycloalkyl or aryl, such as benzyl, trimethylphenyl, ethylphenyl, are in particular, considered as substituents. Linear alkyl radicals having 1 to 18 C atoms, more preferably methyl, ethyl and benzyl, are preferred.
C6-C24-aryl is understood to mean any of the mononuclear or polynuclear aryl radicals having 6 to 24 C atoms, which are known to those skilled in the art, such as phenyl, naphthyl, anthracenyl, phenanthracenyl, and fluorenyl, which may for their part in turn be substituted. Here, alkyl, as well as cycloalkyl or aryl, such as toloyl and methylfluorenyl, are in particular considered as substituents. Phenyl is preferred.
C3-C18-cycloalkyl is understood to mean any of the mononuclear or polynuclear cycloalkyl radicals having 3 to 18 C atoms, which are known to those skilled in the art, such as cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl and the further homologues, which may for their part be in turn substituted. Here, alkyl, as well as cycloalkyl or aryl, such as benzyl, trimethylphenyl, ethylphenyl, are, in particular, considered as substituents. Cyclohexyl and cyclopentyl are preferred.
The radical X stands for the halogens: fluorine, chlorine, bromine and iodine. X preferably stands for chlorine.
The concentration of the organic acid halide in the reaction medium is preferably within the range 1 to 500 ppm, more preferably within the range 10 to 100 ppm. The molar ratio of acid halide to zirconium and/or hafnium is preferably within the range 0.5 to 50, more preferably within the range 1 to 30 and most preferably within the range 2 to 10.
The polymerization of the monomers generally takes place in a cationic manner at temperatures within the range xe2x88x92120xc2x0 C. to +20xc2x0 C., preferably within the range xe2x88x9295xc2x0 C. to xe2x88x9220xc2x0 C., and at pressures within the range 0.1 to 4 bar.
Suitable zirconium halides and/or hafnium halides are, for example, zirconium dichloride, zirconium trichloride, zirconium tetrachloride, zirconium oxydichloride, zirconium tetrafluoride, zirconium tetrabromide and zirconium tetraiodide, hafnium dichloride, hafnium trichloride, hafnium oxydichloride, hafnium tetrafluoride, hafnium tetrabromide and hafnium tetraiodide and hafnium tetrachloride. Zirconium halides and/or hafnium halides having sterically demanding substituents such as, for example, zirconocene dichloride or bis-(methylcyclopentadienyl)zirconium dichloride, are generally unsuitable. Zirconium tetrachloride is preferably utilized. This may be utilized advantageously in the form of a solution in an anhydrous, acid-free alkane or chloroalkane or a mixture of the two, having a zirconium concentration of less than 4 wt. %. It may be advantageous to store (age) the zirconium solution at room temperature or below for a period of from a few minutes to 1,000 hours before utilization. It may be advantageous to carry out this aging with the action of light.
It may, furthermore, be advantageous to utilize mixtures of the catalyst system according to the present invention with conventional catalysts such as AlCl3 and catalyst systems which are preparable from AlCl3, diethyl aluminum chloride, ethyl aluminum chloride, titanium tetrachloride, tin tetrachloride, boron trifluoride, boron trichloride, vanadium tetrachloride or methyl alumoxane, in particular AlCl3 and catalyst systems which are preparable from AlCl3. This combination is also provided by the invention.
When preparing such mixtures, the molar ratio of Lewis acid: zirconium and/or hafnium may be within the range 99:1 to 1:99, preferably within the range 99:1 to 1:1, more preferably within the range 20:1 to 5:1.
The molar ratio of acid halide to zirconium and/or hafnium in the case of such mixtures is preferably within the range 0.5 to 50, more preferably within the range 1 to 30 and most preferably within the range 2 to 10.
It may be advantageous to add to the catalyst system small quantities of water, alcohols, an alkyl halide or halohydrocarbon.
The polymerization may be carried out in both a continuous and also a discontinuous method. In a continuous method, the process is preferably carried out with the following three feed streams:
1. Solvent/diluent+isoolefin (preferably isobutene)
2. Diene (preferably isoprene)
3. Zirconium halide and/or hafnium halide (preferably ZrCl4 in solvent)+organic acid halide.
In a discontinuous method the process may, for example, be carried out as follows:
The reactor, which is pre-cooled to reaction temperature, is charged with the solvent or diluent and the monomers. The initiator together with the acid halide in the form of a diluted solution is then pumped-in such as to allow problem-free removal of the heat of polymerization. The progress of the reaction can be tracked by means of the heat generation. The catalyst solution may also be added portion-wise through a lock.
All operations are carried out under a protective gas. After the end of polymerization the reaction is terminated with a phenolic antioxidant such as, for example, 2,2xe2x80x2-methylene-bis-(4-methyl-6-tert.-butylphenol), dissolved in ethanol.
The process according to the present invention enables high molecular weight isoolefin copolymers to be prepared. The double bonds are determined by the quantity of incorporated diene. The molecular weights (Mv) generally range from 300-1200 kg/mol (depending on the isoprene content and the reaction temperature), the polymers have a very low gel content.
The polymers which are obtainable are suitable for the production of molded bodies of all kinds, in particular tire components, most particularly so-called inner liners, as well as so-called technical rubber goods such as stoppers, damping elements, profiles, films, coatings. For these purposes, the polymers are utilized pure or in mixture with other rubbers such as BR, HNBR, NBR, SBR, EPDM or fluorinated rubbers.
The Examples which follow are provided for the purpose of illustrating the present invention: